A number of bacteria, particularly strains of Acetobacter, can be cultivated to produce bacterial cellulose. In the presence of sugar and oxygen, cells of Acetobacter synthesize cellulose extracellularly in the form of fibrils attached to the cell. The fibrils produced by other cells intertwine to form a hydrophilic network known as a pellicle. Typical processes for the production of microbial cellulose have used static cultivation (GB No. 2131701). The pellicle of microbial cellulose is formed on the air/liquid interface of a motionless and undisturbed culture which is usually contained in shallow trays. This coherent gel-like pellicle is desirable for many uses such as a wound dressing, after processing to remove the cells.
Another method of production of bacterial cellulose uses traditional deep-tank stirred bioreactors. Under this agitated condition, a nonpellicular form of bacterial cellulose is produced. This type of production scheme is highly susceptible to strain instability which is demonstrated by the loss of ability to produce cellulose and gradual overgrowth of the cells (Valla, S. and Kjosbakken, J, Cellulose negative mutants of Acetobacter xylinum, Journal of General Microbiology, 128, 1401-(1982)). Nevertheless, sustained production of reticulated bacterial cellulose under agitated conditions for over 70 hours has been reported (U.S. Pat. No. 4,863,565) using mutagenized and selected strains.
Also, a four-step process for producing microbial cellulose product (U.S. Pat. No. 5,273,891) involves the use of stirred batch culture. The use of airlift fermentors where air bubbles are used to provide mixing instead of impellers, has also been adapted for bacterial cellulose production (Okiyama, A. Shirae, H., Kano, H. and Yamanaka, S., Bacterial cellulose I. Two-stage fermentation process for cellulose production by acetobacter aceti, Food Hydrocolloids, 6, No. 5, 471-477. (1992)). Thus, besides undisturbed static cultivation in trays, no alternative bioreactor that is capable of producing coherent membrane pellicles of bacterial cellulose has heretofore been reported.
Although film bioreactors have been common in the wastewater treatment industry for over 20 years, this type of bioreactor configuration is not employed for the production of a biopolymer. The basic idea of film reactors is to attach the growing cells onto a structural element of the reactor to form a film and then promote cell growth either by flowing nutrients across the film or by moving the film contained in the structural element into a nutrient filled vessel. The most popular of this type of film bioreactor is the trickling filter where the film is stationary and media percolates through the film; and the rotating biological contactor (RBC) where the film is dipped into the medium. One known application of this type of film bioreactor besides wastewater treatment is in the production of filamentous fungi (U.S. Pat. No. 5,246,854). This attached growth biological reactor uses a rotating cylinder to which the filamentous fungi attach while the cylinder is partially submerged in a trough filled with nutrient media. While much has been done to promote the use of these conventional film bioreactors for environmental applications, nowhere has it been previously mentioned that the concept of moving a microbial film into a stationary media can be applied in the production of biopolymers such as extracellular cellulose product.